Hydraulically Operated Clutch

by Roger Garnett
revised: 2/28/92

This page was last updated on 04/27/00

Mechanical Operation

Basic description: When you step on the pedal you push in on a small piston in the clutch master cylinder, which pushes fluid out of the cylinder, and causes a similar piston to move in the slave cylinder. This in turn pushes a lever on the side of the transmission. The lever pushes on the throw-out bearing, which pushes on the springs on the pressure plate, and thus releases pressure sandwiching the clutch disk between the pressure plate and the flywheel. (whew! see diagram.) The flywheel and pressure plate are bolted to the engine, and the clutch disk rides on the transmission shaft. When you step on the pedal, the clutch disk (and transmission shaft) can freewheel, turning independently from the engine. Release the pedal, and everything "locks" together, and the transmission input shaft turns at the same speed as the engine.


                       === Slave Cylinder and lever
               |         |       (some cars- return spring on lever)
               |   |     O
             --| | | \   |
   engine      |======]=[======= transmission shaft
             --| | | /  ^Throw out bearing
               |   |
               |   ^Pressure Plate
       Flywheel^ ^Clutch disk

Note: Many cars use a cable to act on the engagement lever, instead of hydraulics. A few, such as pre-war MG's, utilize a pedal connected directly to a shaft through the transmission. A sudden clutch failure is most often caused by hydraulic failure, (or broken cable). Slow deterioration could be anything. How to determine whether a problem is hydraulic or the clutch itself: Get somebody to activate the pedal. You should be able to see the lever on the slave cylinder move back and forth about 1 to 1 1/2". If not, you have a hydraulic problem. If you have sufficient lever movement, you probably have a clutch problem, and it's time to remove the engine or transmission. The total to replace all the parts (disk, pressure plate, throw-out bearing) may be about $75-150 for most cars. The flywheel might need to be turned, to eliminate warpage, scoring, or glazing, or even replaced if there is cracking. The labour time will be higher if you have to remove the engine to get at the clutch. On some cars, you can just remove the transmission, and for many front wheel drive cars, accessing the clutch is as easy as removing a cover plate or two.

Clutch Hydraulic System Operation

Clutch Master Cylinder

 
                            |-----|
                          |--     --|
                          |         |
                          |         | <- Fluid Reservoir
                          |         |
                          |         |
                          |         |
                 ------------------ -------------------|  Fluid Outlet (pipe)
Pedal Push Rod   |  ||----------|/                      \__________
     ===============||          || <- Piston &           __________ to slave
                 |  ||----------|\    Rubber Seals      /           cylinder ->
                 --------------------------------------|
                  (optional- misc. valving or springs)

Normal Operation:

1) At rest, fluid is free to flow from the hole below the reservoir into the cylinder. (or visa-versa) 2) When you step on the pedal, it moves the push rod/piston assembly. (To the right in diagram.) This first covers the inlet hole, and then proceeds to push fluid through the outlet pipe, to the slave. (Which in turn pushes a piston in the slave, and moves the clutch arm.) 3) When the pedal is released, the piston returns, due to back pressure from the clutch springs on the slave, and (in some cases) a return spring in the master cylinder. Fluid flows back into the master cylinder from the slave/pipe. When the piston is fully retracted, the inlet hole is once again uncovered, and fluid may flow in to make up for any losses. (or out, if the slave has returned farther than it's previous position.)

Possible Master Cylinder Failures:

Front seal fails, or pitting of cylinder wall. Fluid leaks back towards the reservoir instead of pumping out the outlet pipe. This will result in reduced or no slave piston movement. The effects may vary with a borderline seal. (rebuild or replace master cylinder)
Air in system- master, line, or slave cylinder. Air compresses, resulting in reduced piston movement, as above, & soft pedal. (bleed system)
Rear seal fails, fluid leaks out the back of the master cylinder body, possibly onto the drivers foot. (R/R master cylinder)
Piston doesn't return completely due to crud or rough surface. Cylinder remains partially pressurized, and slave doesn't return, leaving clutch in intermediate state. This may leak back slowly. (R/R master cylinder)
Blockage of inlet - shortage of fluid, reduced piston travel, mushy pedal. (R/R master cyl.)
Blockage of outlet -Hard pedal &/or fluid blows back past front seal. (R/R master cylinder)

Clutch Slave Cylinder

                   ++ <- rubber boot
                   +---------------------------|   Fluid Inlet (pipe)
      Push Rod     +               \|-----|    |
      _____________________________||     |     \__________   <-----
     (_____________________________)|     |      __________  from master
      to clutch    +               ||     |     /            cylinder
      <-- arm      +               /|-----|    |=] bleed
                   +---------------------------|   nipple
                   ++                ^piston w/
                                     rubber seal (may also have a return spring

Operation:

1) Rest position. Neutral pressure. 2) Step on pedal- Fluid from master cylinder pressure pushes the slave piston and push rod, (to the left in diagram), which moves the clutch arm, which pushes the throw-out bearing into the pressure plate to disengage the clutch. 3) Release pedal. Fluid pressure from master cylinder is relieved. Spring force of clutch/clutch arm, (and piston return spring, if fitted) pushes the slave piston back to the rest position, which forces the fluid back into the master cylinder.

Failure modes:

-Seal fails, fluid blows past piston (and eventually begins to leak out of the boot). Noticeable as fluid loss in the master cylinder reservoir. Piston travel is reduced or eliminated. (this can result from a torn or rotted seal, or pitted cylinder, etc.)
-Piston seizes or drags. Clutch may be engaged, disengaged, or sluggish, depending on the position and amount of movement of the piston. Caused by corrosion, crud, shredded seal, etc. This may result in a hard pedal, and/or failure of the seal. This could also destroy a weak seal in the master cylinder. (R/R slave cylinder)
-Air in system. Reduced piston travel, clutch won't fully disengage. (bleed system)
-Bad/missing lever or piston return spring. (This varies with design).

Comments:

Cylinder repair kits are cheap. Even a new cylinders can be reasonable. Cylinders may be honed lightly (Be very careful with alloy cylinders!), and should be polished using crocus cloth or fine metal polishing compound.
Remember- USE CASTROL LMA GTX or other DOT4 eq. or better brake fluid, NOT DOT3 in your British car! DOT4 should be changed every 2 years.
DOT5, Silicon is OK too. I'm currently running some long term tests on it, with good results. It should perform much better for a low usage or stored vehicle as it won't absorb water, and therefore should result in very low corrosion levels. DOT4 may be preferable for high use, or competition vehicles.
Brake systems are similar- they just have more cylinders, tubes, and other fittings to deal with.

Pedals and Linkages

The brake, clutch, and accelerator pedals on many British sports cars are located by a shouldered bolt or shaft passing through a simple bushing in the pedal. These bushings are often not lubricated, and can result in worn shafts/ bushings, and a sloppy pedal. An even worse problem, is the linkage to the brake and clutch master cylinder pushrods. This is often just a hole drilled through the pedal body, with a clevis pin joining the pushrod and pedal. Wear at this joint is often excessive, resulting in an ovaled hole in the pedal body, and a narrowed clevis. Due to the mechanical ratios, a small amount of wear at this joint will result in a large reduction in pedal travel. You can check this some by wiggling the pedal up/down, and side to side, but visual inspection is best. The fix is simple. First, remove the pedal assemblies from the car. Fill in the ovaled hole by welding or brazing, and re-drill the hole. This should be a fairly close fit, probably no more than .005" of slop. Ream to fit if necessary. If the push-rod holes are worn, they may be repairable as well, or may be replaced. If the pivot bushings or shafts are worn as well, now's your chance to fix them too. When reassembling, coat the pivot bushings with some good wheel bearing grease. Fit new clevis pins, which only cost about 50 cents. Do not skimp and use a threaded bolt instead, as both it and the pedal will wear away quickly. A couple drops of ATF, or possibly motor oil here completes the job.

Source

Roger_Garnett@cornell.edu)
The Wayward Sports Car Centre

The editor of this page is Bob Haskell.

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